In an exhaust system of an internal combustion engine which performs lean combustion such as in a diesel engine, a NOX catalyst device is arranged to purify NOX in the exhaust gas. One suggested NOX catalyst device satisfactorily stores NOX in the exhaust gas when the air-fuel ratio of the exhaust gas is lean, namely when the concentration of oxygen in the exhaust gas is high. On the other hand, as a regeneration treatment, when the air-fuel ratio of the exhaust gas is made the stoichiometric air-fuel ratio or a rich air-fuel ratio, namely, when the concentration of oxygen is decreased, the NOX catalyst device releases the stored NOX and purifies the released NOX to N2 by reducing materials in the exhaust gas.
However, during the regeneration treatment, N2O (dinitrogen monoxide) can be produced. It is also undesirable to release N2O into the atmosphere. Accordingly, an exhaust purification system of an internal combustion engine is suggested, which is intended to reduce an amount of N2O flowing out from the NOX catalyst device during the regeneration treatment (for example, refer to Japanese Unexamined Patent Publication No. 2004-211676).